1. Technical Field
The present disclosure relates generally to a motor reversing circuit. More specifically, the present disclosure relates to a motor reversing circuit for dc systems which operate at voltage levels greater than 12 volts.
2. Background of the Related Art
H-bridge motor reversing relay arrangements are commonly used in systems which require bi-directional dc motor operation. For example, an H-bridge circuit is employed in automotive systems having bi-directional motors for operating power door locks, power seats, power windows, etc. Typically, bi-directivity is achieved by the H-bridge circuit by using two single pole, double throw (SPDT) relays controlled by a forward and reverse single pole, double throw, center off switch to change the polarity of the voltage inputs to the motor to provide forward and reverse operations.
Each SPDT relay operates between a normally open (N.O.) and a normally closed (N.C.) position for supplying and cutting-off power to the motor. The load is positioned between a movable contact of each SPDT relay with the negative polarity of the voltage source on both N.C. stationary contacts and the positive polarity on both N.O. stationary contacts. Ground is provided to the N.C. contacts such that both sides of the motor windings are grounded when neither relay in the H-bridge circuit is operated.
When one of the relays of the H-bridge circuit is energized, it applies a positive voltage to one side of the motor through one of the SPDT switches (relay contacts) while the other side remains grounded through the N.C. stationary contact of the other SPDT switch (relay contacts). This causes the motor to rotate in a specific direction. If the relay is de-energized the motor discontinues operation and the motor windings are again connected to ground. If the other relay is energized it applies a positive voltage to the other side of the motor through the other SPDT switch and causes the motor to rotate in the opposite direction.
The conventional H-bridge circuit as described above works well for 12-volt dc systems. However, in higher voltage systems, such as 24-volt dc systems, which typically necessitate operation up to 32-volt dc, the H-bridge relay circuit creates an arc between the moveable contact and the N.O. stationary contact of the switch as it opens. The arc is created due to the relatively high current and high potential difference between the moveable contact and the N.O. stationary contact. Further, because the N.O. stationary contact has positive battery voltage and the N.C. stationary contact is at ground potential, a short circuit is created through the arc causing the current flow through the arc to increase, limited only by circuit impedance. The arc continues until it has melted away enough contact material to establish a sufficient gap to extinguish the arc.
Therefore, a need exists for a motor reversing circuit for operating a bi-directional motor at higher voltage levels that does not cause arcing and still provides bi-directional operation of the load.